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Abstract

The E. coli Min system contributes to spatial regulation of cell division by preventing Z ring assembly at cell poles. Critical to our understanding of this spatial regulation by the Min system is the mechanism of action of MinC, an inhibitor of Z ring formation. Even though the Min system has been extensively studied, the molecular mechanism by which MinC antagonizes Z ring assembly is still not very clear, which is the goal of this study. MinC has two functional domains, both of which are able to block cell division in the proper context---MinCn can do so by itself whereas MinCc requires MinD. In this work, we describe the inhibitory mechanism of each domain of MinC on Z ring assembly. First, we show that the septal localization and division inhibitory activity of MinCc/MinD requires the conserved C-terminal tail of FtsZ. Using a genetic screen we identified four mutations in FtsZ that significantly decrease the MinCc/MinD-FtsZ interaction and the toxivity of MinCc/MinD. These mutations are clustered at the conserved C-terminal tail of FtsZ, a region critical for FtsZ-FtsA and FtsZ-ZipA interactions and therefore Z ring assembly. Using this as a clue, we were able to show that the toxicity of MinCc/MinD in blocking division is due to its competition with FtsA and/or ZipA for the tail of FtsZ. In the presence of overexpressed MinCc/MinD, such competition displaces FtsA and/or ZipA from the Z ring to disrupt the integrity and functionality and eventually totally destroy the structure of the Z ring. Second, we studied the interaction between FtsZ and the N terminal domain of MinC. MinCn has been shown to be the anti-FtsZ part of MinC but the detailed mechanism regarding this activity is not known. Previous studies lead to the puzzling observation that MinCn blocks FtsZ polymer sedimentation but does not affect its GTPase. Because the GTPase activity of FtsZ is linked to its polymerization, MinCn is believed to act after the polymerization of FtsZ to shorten FtsZ polymers. Using a similar genetic screen as above, we identified the residues in FtsZ that are critical for the MinCn-FtsZ interaction. These important residues are clustered at the FtsZ dimerization interface, indicating that MinCn attacks FtsZ polymers at the dimer interface. Based on this, a "wedge" model for the action of MinCN on FtsZ is proposed. Collectively, this study encourages us to suggest a more detailed model for how MinC/MinD antagonizes the Z ring formation: MinC/MinD localizes to the Z ring or membrane-associated FtsZ polymers through MinCc/MinD interacting with the conserved C-terminal tail of FtsZ. By directly contacting FtsZ, MinC/MinD prevents Z ring formation in at least two ways: first, MinCc/MinD disrupts the function and structural integrity of the Z ring by interfering with the recruitment of FtsA and/or ZipA; second, this targeting of MinC/MinD to the Z ring brings MinCn in close proximity to FtsZ polymers, which then severs these FtsZ polymers so that the Z ring is completely destroyed. By targeting different regions of FtsZ the two domains of MinC affect different aspects of Z ring formation to achieve synergy in disrupting Z rings. Normally the activity of MinC/MinD is spatially regulated by MinE so that it works only at cell poles to block the formation of any potential polar Z rings. During the course of this study, we discovered another layer of spatial regulation of cytokinesis by MinC/MinD independent of MinE. The accumulated evidence shows that polar Z rings are more sensitive to MinC/MinD than midcell Z rings even in the absence of MinE. In some cases such as in the FtsZ-I374V strain, wild type morphology can be achieved by MinC/MinD without MinE. The mechanism of this differential MinC/MinD sensitivity between polar and midcell Z rings is unknown but it suggests that another layer of spatial regulation of cytokinesis by MinC/MinD exists other than oscillation induced by MinE.